[Docs] Note change to Maintainers.md in DeveloperPolicy (#200315)
This was brought up in a Discord discussion around adding a new target.
Update the documentation to mention Maintainers.md everywhere and remove
references to Maintainers.rst where the former is exclusive.
The only remaining usage of Maintainers.rst is in clang-tools-extra and
we should probably just rewrite it in Markdown especially given it seems
like there is conesnsus to move away from ReST entirely, but we can do
that later.